Importance of Hydrogen Production without Greenhouse Gas Production
Hydrogen as an energy carrier will be one of several key driving forces for increased hydrogen demand in the future. Steam reforming of natural gas, sometimes referred to as steam methane reforming (SMR), is the most common method of producing commercial bulk hydrogen as well as the hydrogen used in the industrial synthesis of ammonia. It is also the least expensive method available for producing hydrogen. The first step of the SMR process involves methane reacting with steam at 750-800° C. to produce synthesis gas, a mixture primarily made up of hydrogen and carbon monoxide. In the second step, known as the water gas shift reaction, the carbon monoxide produced in the first reaction is reacted with steam over a catalyst to form hydrogen and carbon dioxide. This process occurs in two stages, the first stage involves a high temperature shift at 350° C. while the second stage involves a low temperature shift at 190-210° C. The main disadvantage of the SMR process is the production of carbon dioxide, a greenhouse gas that is having a negative impact on global climate. Thus, other avenues for hydrogen production are being sought that do not generate greenhouse gases.
Electrochemical Production of Hydrogen
Hydrogen has also been produced commercially through the electrolysis of water. Traditionally, the electrochemical production of hydrogen has involved the electrolysis of water in alkaline solutions according to the following equation:H2O(l)→H2(g)+½O2(g)  (1)
The reversible cell potential for this reaction is −1.23 V. This implies that the free energy change for this reaction is positive. Thus, in order for this reaction to proceed as written, energy must be added to the system. The standard free energy change, ΔG°, for this reaction is 237 kJ mol−1. To drive this reaction at acceptable rates a potential equal to about 1.8 V is required. The additional electrical energy required for this reaction results from activation overpotentials as well as from ohmic losses within the cell. One advantage of water electrolysis is that no greenhouse gases are produced. However, since the cell potential is large, hydrogen produced by the direct electrolysis of water is expensive. Because of the high cost of water electrolysis, other routes for producing hydrogen are being sought.
The heat that must be supplied to the system to produce hydrogen and oxygen according to Equation (1) is given by the standard enthalpy change, ΔH°, which is equal to 286 kJ mol−1. This energy can be supplied to the system in the form of heat or it can be supplied to the system by a combination of both heat and electricity. As the heat added to the system increases, the required amount of electricity decreases. Alternatively, as the amount of heat added to the system decreases, the required amount of electricity increases. This is the basis of hybrid thermochemical electrolytic water splitting cycles which use both heat and electricity to supply the total energy requirement. These cycles involve two or more reactions with at least one reaction being an electrochemical reaction. Overall, in a hybrid thermochemical electrolytic cycle, water, heat and electricity are consumed while hydrogen and oxygen are produced as reaction products. The process forms a closed loop with all intermediate chemicals being recycled. It should be emphasized that in these hybrid thermochemical electrolytic water splitting cycles, hydrogen and oxygen may or may not be produced electrochemically. One advantage of hybrid thermochemical electrolytic cycles is the electrical energy requirement of the electrochemical step is considerably less than it is for direct water electrolysis.
Proposed Generation IV Very High Temperature Reactor (VHTR) designs contemplate nuclear reactors that will be capable of supplying process heat at temperatures of up to 900° C., which is sufficient to supply the heat required by the chemical reactions of hybrid thermochemical electrolytic cycles. Solar heat is another non-carbon based option for hybrid thermochemical electrolytical cycles. Since these heat sources do not generate greenhouse gases, and since hydrogen and oxygen are the net reaction products of a thermochemical water splitting cycle, thermochemical cycles are environmental friendly processes that do not generate greenhouse gases.
Hybrid thermochemical electrolytic water splitting cycles that can be carried out at temperatures below 600° C. are of interest to Canada. Canada is participating in the Generation IV International Forum (GIF) on the development of advanced hydrogen-production processes using heat from nuclear reactors operating at temperatures in the range of 500 to 900° C. Since Canada's interests are mainly in pressure-tube reactors, current plans and efforts are directed towards development of a Super Critical Water Reactor (SCWR), which will be a nuclear reactor that is capable of supplying process heat at temperatures up to about 625° C. Therefore, Canada is most interested in high temperature hydrogen production processes that can operate at temperatures at the lower end of the temperature range considered for VHTRs.
Copper Chlorine Thermochemical Cycle
The copper-chlorine (Cu—Cl) thermochemical cycle is a hybrid process that uses both heat and electricity to carry out a series of chemical and electrochemical reactions with the net reaction being the splitting of water into hydrogen and oxygen.
The Cu—Cl cycle has two variations, which are known as the four-step process and the five-step processes. The four-step process can be summarized as follows:
TABLE 1The Four-Step Cu—Cl Thermochemical CycleStepReaction12CuCl(aq) + 2HCl(aq) → H2(g) + CuCl2(aq)2CuCl2(aq) → CuCl2(s) (drying step)32CuCl2(s) + H2O(g) → Cu2OCl2(s) + 2HCl(g)4Cu2OCl2(s) → 2CuCl(l) + ½O2(g)
If the reactions given by steps 1-4 above are added together, the following net reaction results:H2O(g)→H2(g)+½O2(g)
In the four-step Cu—Cl cycle, a chemical species that is consumed in one reaction, such as HCl in Step 1, is produced in a different reaction, which is Step 3 for HCl. Thus, all of the chemicals are recycled expect for water, hydrogen and oxygen which is consistent with the net reaction being the splitting of water.
In a paper entitled, “Generating Hydrogen Using a Low Temperature Thermochemical Cycle”, by M. A. Lewis, M. Serban and J. Basco, Proceedings of the ANS/ENS 2003 Global International Conference on Nuclear Technology, New Orleans 2003, Argonne National Laboratory identified the Cu—Cl cycle as one of the most promising lower temperature cycles for hydrogen production. The Cu—Cl process is of interest to Atomic Energy of Canada Limited (AECL) because all of the chemical and electrochemical reactions can be carried out at temperatures that do not exceed about 530° C. This means that the heat requirement of this process can be supplied by the Generation IV SCWR that is being developed by AECL.